1. Field of the Invention
The present invention relates to a light emitting element having a high level of brightness and a manufacturing method for the same.
2. Description of the Related Art
An optical power output of a semiconductor light emitting element (LED) is determined by a product of an internal quantum efficiency representing a conversion efficiency of an injection current to photons, and an extraction efficiency for the photons emitted from the semiconductor passing through a molded resin or air. In order to improve the extraction efficiency and increase the optical power output of the LED, many methods have been proposed, such as tilted side surfaces, roughened side and top surfaces, microlenses, anti-reflection films and the like.
A basic idea for improving the extraction efficiency can be described as follows. The photon extraction efficiency depends on a difference in the refractive index of a semiconductor layer and a surrounding medium. For example, when a periphery of the semiconductor layer is encapsuled with an epoxy resin, the photons with an incident angle less than 27 degrees regarding to the normal of a surface of the semiconductor layer may be extracted from the surface. In the case of commercial LEDs, an optical design is adopted so that the photons with the incident angle of 27 degrees or greater are reflected at an interface between the semiconductor layer and the epoxy resin. Generally, the reflected photons may disappear by being absorbed in the semiconductor layer, or as a result of multiple reflections, only a portion of the reflected photons may be extracted. Accordingly, the extraction efficiency for the LED chip encapsuled with the epoxy resin may become poor.
A method to increase the extraction efficiency may be provided by using the tilted side surfaces so as to reduce the incident angle of the photons emitted from a light emitting layer of the LED to the interface between the semiconductor layer and the surrounding medium. Alternatively, by using a method for roughening the surfaces of the semiconductor layer so as to bring about a random angular distribution for the interface between the semiconductor layer and the surrounding medium, the photons can be extracted more efficiently using a light scattering effect.
For example, with regard to the tilted side surfaces, in Japanese Patent Applications Laid-open Nos. Hei 10-341085 and 2001-68743, the extraction efficiency is improved by fabricating an LED chip with a trapezoidal shape. In addition, similar proposals regarding the roughened side and top surfaces are disclosed in, for example, Japanese Patent Applications Laid-open Nos. 2000-196141, Hei 10-200162 and 2000-299494. Furthermore, an example of an actual method in making an LED has been given using an electron beam lithography to form the top surface of a GaAlAs LED into a conical shape (refer to M. Ishimori, et al., “Extended Abstracts of The 48th Spring meeting, 31a-ZW-10”, The Japan Society of Applied Physics and Related Societies, 2002, p. 997).
In addition, the extraction efficiency may be improved by using a graded index layer formed on the surfaces of the semiconductor layer, that continuously changes the refractive index from the semiconductor layer to the surrounding medium to prevent optical refraction. In fact, it is difficult to form such a layer having the continuously changing refractive index on the surfaces of the LED. Nevertheless, it is possible to improve the extraction efficiency using the graded index layer by roughening the surfaces of the LED to form small irregularities approximately equal to or smaller than the emitted wavelength.
As described above, by tilting and roughening the side surfaces of the LED it is possible to improve the extraction efficiency, however the following new problems arise
(1) When the tilted side surfaces of a GaP substrate having a crystal orientation correspond or close to a (100) plane, are roughened, due to a relationship with the crystal orientation, only two of the four side surfaces are capable of being roughened and obviously the improvement in the extraction efficiency is less than with all four side surfaces being roughened. Therefore it is difficult to say that an inherent property of the internal quantum efficiency of the light emitting layer is sufficiently exhibited.
(2) Reliability for the LED chip may be decreased due to a nonuniform stress of the molded resin. In other words, in a trapezoidal LED chip with the tilted side surfaces molded with a resin such as an epoxy, when a top surface electrode is formed having a larger surface area than a bottom surface electrode, an upward force is applied to the LED chip from the resin. In the opposite case, where the top surface electrode is smaller than the bottom surface electrode, a downward force is applied. Such nonuniform stress exerted on the LED chip causes strain and cracks, as well as a remarkable loss in the long-term reliability of the LED chip. In addition, excessive stress on the LED chip causes peeling and breakage of the LED chip. While in rectangular solid LED chips the stresses with the molded resin are balanced out by canceling the stresses applied to opposite side surfaces, since shape symmetry cannot be assured, the stresses generated with the molded resin are extremely difficult to thoroughly eliminate in a trapezoidal LED chip.